1. Field
Example embodiments relate to an optical interconnection system that converts a two-level signal into a three-level signal and transmits and receives the three-level signal.
2. Description of the Related Art
In high speed semiconductor devices, there are limitations to the speed of signal transmission due to the use of electrical wires and to large capacity high speed signal transmission due to cross-talk between complex wirings, limited packing density, electromagnetic interference (EMI), etc. To address these problems, optical interconnection techniques using light for transmitting signals between semiconductor devices has drawn attention.
In particular, as the speed of operation of semiconductor devices increases, testing systems for testing the semiconductor devices also require high speed operation. An optical interconnection technique that transmits signals using light between a test head to which a test signal is applied and a device under test (DUT) in semiconductor testing systems has been proposed.
Generally, the signal input/output level of an optical interconnection system includes a high level signal and a low level signal which may correspond to “on” and “off” signals. As such, the optical interconnection system transmits and receives a two level signal.
In addition to the high and low input/output signal levels, semiconductor memory devices further require a reference signal intermediate to the high level signal and the low level signal.
FIG. 1 illustrates a conventional optical interconnection system 10 that transmits a two-level signal. The optical interconnection system 10 may include a transmitting unit 12 that transmits an optical signal, a receiving unit 14 that receives the optical signal, and/or an optical waveguide 16 through which the optical signal is transmitted and received. The transmitting unit 12 outputs the optical signal in response to an electrical input signal, and the receiving unit 14 outputs an electrical signal in response to light received from the optical waveguide 16.
FIG. 2 is a waveform of an output voltage in a conventional optical interconnection system.
Referring to FIG. 2, an optical signal includes a two-level signal, for example, a high level voltage signal VH and a low level voltage signal VL.
FIG. 3 illustrates a three level signal waveform output from a semiconductor memory according to conventional art.
FIGS. 4A and 4B are waveforms of a voltage output from the conventional optical interconnection system of FIG. 1.
As illustrated in FIG. 3, a signal output from a semiconductor memory may include, in addition to the high level signal and the low level signal, an intermediate level signal, for example, a reference signal VREF which is between the high level signal and the low level signal. That is, there are three signal levels.
Thus, in order to optically interconnect a signal in a semiconductor memory a system that transmits and receives three-level signals is required.
When a signal with a waveform of FIG. 3 is input to an optical interconnection system which operates with conventional two-level signals, an incorrect voltage of the reference signal is output. If a threshold voltage VTH of a signal input to the transmitting unit 12 is higher than the reference voltage VREF of FIG. 3, then, as illustrated in FIG. 4A, the signal VREF input to the transmitting unit 12 is output as a low voltage signal VL from the receiving unit 14. Thus, an input signal with the waveform of FIG. 3 is output as the waveform of FIG. 4A. The threshold voltage VTH denotes a voltage that differentiates a low voltage signal VL and a high voltage signal VH in a two-level optical interconnection system.
FIG. 4B illustrates an output waveform when the threshold voltage VTH is lower than the reference voltage VREF. Since the reference voltage VREF is higher than the threshold voltage VTH, the reference voltage VREF input to the transmitting unit 12 is output as a high voltage signal VH from the receiving unit 14.